AGL-CS-087 — Runway Sign Illumination Failure
The AGL Knowledge Hub | Engineering Case Study Series. A high-priority runway sign investigation where moisture-induced LED driver failure caused complete loss of mandatory instruction sign illumination — from CCR verification through transformer testing, sign input analysis, driver diagnostics, moisture inspection, seal restoration, and photometric verification.
Important: This is an engineering simulation created for training. It is not presented as an actual airport incident. All readings, values, dates and outcomes are illustrative, and no airport, organisation, person or manufacturer is identified or implied.
1Case Study Overview
During routine night operations, air traffic control reported that the runway holding position sign at Taxiway B was completely dark.
Pilots reported difficulty identifying the runway holding point, particularly during low-visibility taxi operations.
The adjacent taxiway edge lights, stop bar lights, and centerline lights were operating normally, and the associated Constant Current Regulator (CCR) showed normal operation.
Because the failed sign was a mandatory instruction sign protecting runway entry, a high-priority investigation was initiated covering:
- Isolating transformer operation.
- Sign input power.
- LED driver electronics.
- Internal LED modules.
- Secondary cable integrity.
- L-823 connectors.
- Water ingress.
- Photometric and color compliance.
The investigation identified failure of the internal LED driver module caused by moisture ingress into the sign enclosure, resulting in complete loss of sign illumination.
This case demonstrates that runway sign illumination failures are operationally critical events that require immediate diagnosis and restoration because they directly affect runway-entry control and runway-incursion prevention.
2Learning Objectives
After completing this case study, the learner should be able to:
- Diagnose runway sign illumination failures.
- Verify runway-sign power circuits.
- Test isolating transformers.
- Investigate LED driver failures.
- Assess moisture-related electronic damage.
- Restore mandatory-sign illumination.
- Perform operational and photometric verification.
3System Background
A runway mandatory sign system includes:
- Primary series circuit.
- Isolating transformer.
- Secondary cable.
- L-823 connectors.
- Sign power input.
- LED driver module.
- LED arrays.
- Optical diffuser.
- Color filter system.
- Weatherproof enclosure.
Common failure causes include:
- LED driver failure.
- Moisture ingress.
- Transformer failure.
- Secondary cable damage.
- Connector corrosion.
- Seal deterioration.
- Internal wiring failure.
- Electronic component degradation.
Mandatory signs are critical for runway holding-point protection.
4Problem Statement
The reported condition was:
- Runway holding-position sign OFF.
- Adjacent AGL equipment operating.
- CCR healthy.
- No external mechanical damage.
- Reduced runway holding-point conspicuity.
Has the runway sign lost electrical power, or has an internal sign component failed due to environmental deterioration?
5Initial Observations
| Sign illumination | OFF |
| Adjacent AGL lights | ON |
| CCR operation | Normal |
| External damage | None |
| Sign housing | Slight moisture visible |
The failure appeared localized to the sign assembly.
6Initial Engineering Hypotheses
The investigation considered:
CCR output failure.
Isolating-transformer failure.
Secondary-cable failure.
LED driver failure.
Water-ingress-related internal failure.
7Runway Sign Failure Principle
Runway signs are safety-critical visual control devices.
8Test Equipment
The investigation used:
- Digital multimeter.
- Insulation resistance tester.
- Transformer test equipment.
- LED driver analyzer.
- Moisture detection equipment.
- Thermal imaging camera.
- Portable photometer.
- ICAO Annex 14 sign-reference requirements.
9Investigation
Step 1: Verify CCR Operation
Results:
| Output current | Normal |
| Output voltage | Normal |
| Regulation | Stable |
| CCR alarms | None |
The CCR was healthy.
Step 2: Verify Transformer Output
Measurements showed:
- Primary current: Normal
- Secondary voltage: Present
The isolating transformer was healthy.
Step 3: Secondary Cable Verification
Results:
| Continuity | Good |
| Connector condition | Good |
| Insulation | Acceptable |
The field cable system was healthy.
Step 4: Sign Input Voltage
Measured voltage at the sign input terminals:
Correct input voltage present
Power was reaching the sign enclosure.
Step 5: Internal Driver Test
Measured driver output:
0 V DC
Expected:
36 V DC
The driver module was not functioning.
Step 6: Internal Inspection
Inspection revealed:
- Moisture droplets.
- Condensation.
- Corroded driver PCB.
- Electrolytic capacitor damage.
- Oxidized electronic components.
The enclosure seal had deteriorated.
Step 7: LED Module Verification
Using a regulated DC test supply:
LED arrays illuminated normally.
The optical system remained healthy.
10Root Cause
The runway sign illumination failure was caused by moisture-induced failure of the internal LED driver module, resulting from:
- Enclosure seal deterioration.
- Water ingress.
- PCB corrosion.
- Driver electronic failure.
The transformer, secondary cable, connectors, and LED arrays remained healthy.
The primary failure mechanism was environmental degradation of the driver electronics.
11Corrective Action
The maintenance team:
- Isolated the sign.
- Replaced the LED driver module.
- Replaced enclosure seals.
- Removed moisture contamination.
- Cleaned internal components.
- Verified LED array operation.
- Measured sign luminance.
- Verified red background and white legend color compliance.
- Conducted operational observation.
- Updated preventive-maintenance procedures.
12Post-Repair Verification
Driver replacement restored full runway-sign operation.
13Root-Cause Analysis
Runway sign dark
Healthy
Healthy
Present
Absent
Moisture damage found
Sign retested
Moisture-induced LED driver failure
Replace driver and restore enclosure integrity
14Why This Case Was Important
Runway sign failures can cause:
- Reduced holding-point recognition.
- Increased runway-incursion risk.
- Low-visibility operational degradation.
- Increased pilot workload.
- ICAO compliance concerns.
- Potential operational restrictions.
Mandatory signs are critical runway-protection devices.
15Engineering Lessons
Runway sign failures have higher operational priority than general taxiway signs.
Runway sign failures have higher operational priority than general taxiway signs.
Moisture ingress is a major cause of sign electronic failures.
Moisture ingress is a major cause of sign electronic failures.
Transformer output and driver output must be tested separately.
Transformer output and driver output must be tested separately.
Enclosure seal integrity directly affects sign reliability.
Enclosure seal integrity directly affects sign reliability.
Photometric and color verification are essential after repair.
Photometric and color verification are essential after repair.
16Engineer's Decision Point
You observe: Runway holding-position sign completely dark. Adjacent AGL lights operating. CCR healthy. Sign input voltage present. What should you investigate first?
17Knowledge Check
18Interview Questions
19Frequently Asked Questions
Quick answers about runway sign illumination failure and moisture-induced driver failure.
Q1What is AGL-CS-087?
AGL-CS-087 is an engineering training case study. It covers a dark runway holding-position sign at Taxiway B, where moisture damaged the internal LED driver. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2Why are runway sign failures operationally critical?
They are safety-critical visual control devices. A dark holding-position sign protects runway entry, and pilots had difficulty identifying the holding point, especially in low visibility.
Q3What did the checks on the supply show?
The CCR, transformer output and secondary cable were healthy, and correct input voltage reached the sign. The driver output was 0 V DC against an expected 36 V DC.
Q4What did internal inspection find?
Moisture droplets, condensation, a corroded driver PCB, electrolytic capacitor damage and oxidized components, with a deteriorated enclosure seal.
Q5Were the LEDs damaged?
No. The LED arrays lit normally on a regulated DC test supply, so the fault was in the driver.
Q6What was the root cause of AGL-CS-087?
Moisture-induced failure of the internal LED driver module, caused by enclosure seal deterioration, water ingress and PCB corrosion.
Q7How was the sign restored?
The team replaced the LED driver module and enclosure seals, removed moisture and cleaned internal components. They also verified LED operation, measured luminance, checked red background and white legend colour compliance, conducted operational observation and updated preventive-maintenance procedures.
Q8What is the difference between this case and AGL-CS-086?
Both ended in driver failure. Here the driver failed because moisture entered through a deteriorated enclosure seal, so the seals were replaced as well as the driver.
Q9What is the key lesson from this case?
Restore runway signs immediately and address the environmental cause. Replacing only the driver without sealing the enclosure invites a repeat failure.
Engineering & Expert Notes
Give Mandatory Signs High Repair Priority
PriorityA dark mandatory sign affects safety. Plan the response accordingly.
Replace Enclosure Seals During the Repair
SealingThe new driver will fail again if water still gets in.
Test Transformer Output and Driver Output Separately
MethodSeparate readings locate the fault.
Add Seal Inspection to the Routine Programme
PlanningCheck seals before they fail.
20Case Study Conclusion
Runway sign illumination failures are safety-critical visual-guidance failures that are commonly caused by moisture-induced LED driver degradation, and systematic verification of transformer output, sign input voltage, driver performance, enclosure integrity, photometric performance, and ICAO color compliance provides the most effective method for restoring reliable runway holding-position guidance.
Systematic electrical diagnosis, moisture investigation, driver replacement, seal restoration, photometric verification, and operational validation can rapidly identify runway-sign failures and restore ICAO-compliant mandatory-sign performance.
In AGL engineering, a dark runway sign is not simply an unlit display; it is a missing runway-control instruction that can directly affect ground safety and runway-incursion prevention.
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